I suspect it's probably due to the character/skin disabling. This may not be a bug and just something I'm derping out on, but it's fairly repeatable as I've been unable to save after several tries. For the record, I play with Mario, Luigi, and Bass disabled, and only one active skin for each of the remaining seven. Also, all of the map skins except the original SMB are disabled. Another possible cause may be the weapon customization; I set Bill's first weapon to the Flare gun and Simon's second one to Holy Water, so that may be it, too.